Diversity type receiver apparatus and receiving method

ABSTRACT

The diversity receiving apparatus of the present invention can arbitrarily determine the master branch which is the reference to synchronize the output timing among the symbols of the signals received by a plurality of branches depending on the receiving condition. This structure allows the master branch to be replaced with another branch in excellent receiving condition when the master branch gets into bad receiving condition or when the antenna signal wire on the master side comes off or breaks, thereby achieving the selection and synthesis of carriers.

TECHNICAL FIELD

The present invention relates to an apparatus and method for diversityreception of data signal sequences which are modulated and demodulatedby a digital multilevel modulation system.

BACKGROUND ART

Diversity reception is often used for stable reception of terrestrialbroadcasts in mobile devices such as cars and mobile phones.

Diversity reception systems include space diversity systems which usedifferences in spatial arrangement between transmitting and receivingantennas; time diversity systems which transmit the same signal aplurality of times to increase the chance of reception; frequencydiversity systems which transmit the same signal in a plurality offrequency bands so as to stabilize the reception in one of the frequencybands by using the property that different frequency bands havedifferent phasing probabilities; and polarization diversity systemswhich use differences in polarization properties between transmittingsignals.

Of these types of diversity systems, the time diversity, frequencydiversity and polarization diversity systems require the transmittingside to transmit the same information either a plurality of times or byusing a plurality of means. Therefore, for the purpose of improving theproperty of receiving terrestrial broadcasts, space diversity systems,which can be achieved by changing the reception style on the receivingside, are more used for the effective use of limited frequency resourcesthan the other types of diversity systems.

In a space diversity system that is in actual use, when an analog TVbroadcast is received in a moving car having a plurality of antennas,the signal having the highest received signal level is selected fromamong the plurality of received signals.

For the meantime, broadcasting digitalization is in progress; forexample, Japan and Europe have adopted an Orthogonal Frequency DivisionMultiplexing” (hereinafter, OFDM) transmission system as a digitalterrestrial TV broadcasting system. The OFDM transmission system isdescribed, for example, in lines 2 to 9 of “Overview of the ISDB-Tsystem” on page 8, chapter 2 of “Transmission System for DigitalTerrestrial Television Broadcast” ARIB STD-B31 Version 1.1 published byARIB (Association of Radio Industries and Businesses).

In a case where a space diversity reception system is applied to theOFDM-modulated signals described above, signals are received by aplurality of antennas, and are separately subjected to A/D conversion,synchronous detection, FFT calculation and demodulation. Consequently,each of the signals received by the plurality of antennas is formed intoa modulated OFDM signal consisting of a larger number of carriers.

In space diversity reception systems, it is most effective to processsignals received by a plurality of antennas OFDM-carrier byOFDM-carrier. More specifically, either the optimum signal is selectedfrom among the signals received by a plurality of antennas OFDM-carrierby OFDM-carrier, or the signals received by the plurality of antennasare synthesized OFDM-carrier by OFDM-carrier.

In the aforementioned diversity reception system which demodulates thesignal components in the OFDM carriers of received OFDM signals, andselects and synthesizes the same number of OFDM carriers as the numberof antennas, the selection and synthesis must be performed by using thesame data.

However, in a mobile reception environment, it is expected that aplurality of the receiving antennas receive signals that have passedthrough different propagation paths, so that the plurality of signalsreceived are not necessarily obtained at the same timing.

Each OFDM signal contains a signal to eliminate the interference fromthe preceding and subsequent symbols, the signal being is called a guardinterval. The signal in the guard interval is a copy of part of theeffective symbol period of the OFDM signal, so that signal continuity ismaintained between the guard interval and the effective symbol period.Therefore, when a received signal in the time domain is converted to asignal in the frequency domain, a signal corresponding to one symbolperiod of the OFDM signal is extracted from the OFDM symbol period (theguard interval+the effective symbol period), and is subjected tofrequency conversion.

The signal corresponding to one symbol period of an OFDM signal isextracted by choosing a period least affected by the interference fromthe preceding and subsequent symbols. This may cause the selected symbolperiod to be different depending on the delay time and magnitude of thedelayed wave contained in each OFDM signal. As a result, OFDM signalsare subjected to frequency conversion at different timings, andconsequently, demodulation units may receive signals at differenttimings.

In a diversity reception system which selects and synthesizes OFDMsignals carrier by carrier, one of a plurality of branches is a masterbranch, and the other branch is a slave branch. In this system, thesynchronous timing of the OFDM symbol of the master branch is firstdetected (for example, detected by notifying a processing unit at alater stage of the symbol head by using a pulse signal). Then, thesynchronous timing of the OFDM symbol detected from the slave branch issynchronized with the synchronous timing of the OFDM symbol of themaster branch. Thus, the timing of the signal of the master branch andthe timing of the signal of the slave branch are synchronized inselecting and synthesizing the carriers.

FIG. 4 is a block diagram showing the structure of a conventionaldiversity receiving apparatus.

In FIG. 4, quadrature detection units 404 a and 404 b each detect anOFDM signal; calculate the frequency error between the carrier frequencyof the transmitting signal and the frequency reference signal of thediversity receiving apparatus; correct the frequency of the frequencyreference signal of the diversity receiving apparatus, and determine thetransmission mode of the OFDM signal and the length of the guardinterval. Quadrature detection units 404 a and 404 b also each extract asignal in the effective symbol period from the OFDM signal and output itto FFT calculators 405 a and 405 b, respectively. Quadrature detectionunits 404 a and 404 b also each output a signal indicatingsynchronization establishment.

FIG. 5 shows the structure of a conventional OFDM symbol period.

As shown in FIG. 5, the OFDM symbol period contains a signal at its headas the signal in the guard interval. The signal at its head is a copy ofthe signal at the tail of the effective symbol period.

Since the signal in the guard interval is a copy of the signal in theeffective symbol period, the signal continuity is maintained between theguard interval and the effective symbol period. This allows a signalhaving the same length as the effective symbol period to be extractedarbitrarily from a combined period of the guard interval and theeffective symbol period, and to be outputted to FFT calculators 405 aand 405 b.

However, in reality, OFDM signals are affected by the transmission pathbetween a transmitting antenna and a receiving antenna. In a multipathenvironment, for example, an OFDM signal is interfered by the precedingand subsequent OFDM signals. Therefore, it is necessary to select as theeffective symbol period a period which is interfered as little aspossible by the preceding and subsequent OFDM signals.

One approach to extract a signal in the effective symbol period from anOFDM signal is to determine the optimum period through the calculationof the correlation between the signal in the guard interval and thesignal in the effective symbol period by making use of the property thatthe signal in the guard interval is a copy of the signal in theeffective symbol period.

FFT calculators 405 a and 405 b each perform FFT calculation of thesignal in the effective symbol period received from quadrature detectionunits 404 a and 404 b, respectively, and output FFT calculation results.The FFT calculation results are determined by the number of FFT pointsin the FFT calculation.

The number of FFT points is larger than the number of carriers in anOFDM signal. For example, when an OFDM signal is made up of severalthousands of OFDM carriers, FFT calculators 405 a and 405 b perform8192-point FFT calculation. As a result of this FFT calculation, 8192complex signals are outputted as the calculation results where eachcomplex signal indicates one OFDM carrier signal.

FFT calculators 405 a and 405 b each output 8192 FFT calculation resultssequentially, for example, in order of descending or ascending OFDMcarrier frequencies. FFT calculators 405 a and 405 b also each output atiming signal in such a manner that it is possible to determine to whichOFDM carrier each FFT calculation result corresponds at a process at alater stage.

Demodulation units 406 a and 406 b each receive the FFT calculationresults of the OFDM signal from FFT calculators 405 a and 406 b,respectively.

First of all, demodulation units 406 a and 406 b each extract anddemodulate a TMCC (Transmission and Multiplexing Configuration andControl) signal inserted in the OFDM signal. As a result, demodulationunits 406 a and 406 b obtain various parameters of the OFDM signal.

The various parameters of the OFDM signal include information such astransmission mode, carrier modulation system, coding rate, hierarchicalinformation, time interleave length and symbol number, which areessential to subject the OFDM signal to a demodulation process, adeinterleave process and an error correction process.

Then, demodulation units 406 a and 406 b each extract pilot signalspreviously embedded in the OFDM signal from the signal as the FFTcalculation results, and estimates the transmission characteristics ofthe propagation path through which the received OFDM signal haspropagated.

Then, the FFT calculation results are divided by the estimatedtransmission characteristics so as to perform a demodulation process.

Demodulation units 406 a and 406 b also each receive the timing signalfrom FFT calculators 405 a and 405 b, respectively, and output thetiming signal by delaying it by a processing delay time in thedemodulation units. Separately from the timing signal, demodulationunits 406 a and 406 b also each output information about the electricpower and amplitude of the OFDM carrier containing the OFDM signal fromthe estimated results of the transmission characteristics.

Synthesis unit 407 receives complex signals as the demodulation resultsfrom demodulation units 406 a and 406 b. Synthesis unit 407 alsoreceives the timing signals, separately from the complex signals.

The following will describe a case where signals are received by usingantenna 401 a through demodulation unit 406 a as the master branch, andantenna 401 b through demodulation unit 406 b as a slave branch.

Synthesis unit 407 receives the complex signals as the demodulationresults from each of the master branch and the slave branch, andperforms a synthesizing process of the complex signals of thesebranches. The synthesis must be performed between complex signals havingthe same symbol and the same OFDM carrier. This makes it necessary todetect the time difference between a signal of the master branch and asignal of the slave branch from the timing signals received fromdemodulation units 406 a and 406 b, and to correct the time difference.

The cause of the time difference between a signal of the master branchand a signal of the slave branch will be described as follows with FIG.6.

FIG. 6 is a view to explain the extraction of an OFDM signal in theeffective symbol period by each of quadrature detection units 404 a and404 b, and shows the OFDM signal of the master branch and the OFDMsignal of the slave branch. In FIG. 6, the horizontal directionrepresents time.

When the master branch and the slave branch determine the timing ofextracting the respective OFDM signals independently of each other, asshown in FIG. 6, the master branch and the slave branch may extract theOFDM signals at different timings from each other. Unless either FFTcalculators 405 a and 405 b or demodulation units 406 a and 406 bperform timing adjustment, the difference in the timing of extractingthe OFDM signals causes synthesis unit 407 to receive the OFDM signalsat different timings between the master and slave branches.

The aforementioned timing difference can be accommodated by temporarilystoring the signals of the master and slave branches in a memory or thelike, and taking them out at a time when both signals of the master andslave branches can be securely obtained. For example, the maximum timingdifference in extracting the OFDM signals is regarded as the length ofthe guard interval, and the signal of the master branch is taken out bydelaying it by the length of the guard interval. At the same time as thesignal of the master branch is taken out, the signal of the slave branchhaving the same OFDM carrier is taken out.

Synthesis unit 407 synthesizes the complex signals received fromdemodulation units 406 a and 406 b.

In the synthesis, synthesis unit 407 a performs a weighting process tothe complex signals and sums them up based on the electric powerinformation and amplitude information of the OFDM carriers containingthe received signals, which have been calculated by demodulation units406 a and 406 b. The electric power information and amplitudeinformation may be quantized values. The electric power information andamplitude information used in the weighting process are also required tobe subjected to the same delay adjustment as the complex signals.

However, a system like this where one of a plurality of branches is themaster branch and the other is a slave branch has the followinginconvenience. When the synchronous timing of the OFDM symbol of themaster branch cannot be detected because the master branch gets intoextremely bad receiving condition, or because of the coming off orbreakage of the antenna signal wire on the master side, even if theslave branch is in good receiving condition, it becomes impossible tosynchronize the timing between the master branch and the slave branch.This makes it impossible to select or synthesize carriers.

The cause of this inconvenience is as follows. In synthesis unit 407shown in FIG. 4, in order to adjust the timing difference between thesignal from the master branch and the signal from the slave branch, thesignal from the master branch is delayed for a certain time, and thesignal from the slave branch which is already delayed separately istaken out at the same timing as the signal from the master branch.

When the receiving condition of the master branch is too deteriorated toextract an OFDM signal corresponding to the effective symbol, thesynthesis unit cannot receive the timing signal which is used as thereference. This makes it difficult to synthesize signals even when theslave branch is in good receiving condition.

For this reason, adopting a diversity receiving system worsens thereceiving condition.

When the synchronous timing of the OFDM symbol of the master branchcannot be detected, it is also impossible to obtain information such asthe transmission parameters of a signal being received. This makes itimpossible, for example, to perform error correction in an errorcorrection unit at a later stage.

SUMMARY OF THE INVENTION

In order to solve the aforementioned problems, the diversity receivingapparatus of the present invention includes: a plurality of demodulationunits each provided for each of the plurality of branches, the pluralityof demodulation units each outputting complex information indicating thesignal point of each of the signals received by the plurality ofbranches; a master branch determination unit for determining the masterbranch which is used as the reference in synchronizing the output timingamong the symbols of the signals received by the plurality of branches,and for outputting a signal indicating the branch determined to be themaster branch; a timing adjustment unit for receiving the signalindicating the branch determined to be the master branch from the masterbranch determination unit, and for adjusting the timing of synthesizingthe signals received by the plurality of branches by synchronizing theoutput timing between the symbol of the complex information receivedfrom the demodulation unit of the master branch and the symbol of thecomplex information received from the demodulation unit of a branchother than the master branch out of the plurality of antennas; and asynthesis unit for synthesizing the signals received by the plurality ofbranches by using the complex information that has been timing-adjustedby the timing adjustment unit.

The diversity receiving apparatus of the present invention allows, whenthe master branch gets into bad receiving condition or when the antennasignal wire on the master side comes off or breaks, to replace themaster branch with an optimum branch so as to synchronize the timingbetween the master branch and the slave branch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a structure of a diversity receivingapparatus of a first embodiment of the present invention.

FIG. 2 is a view showing a method for restoring bit data in the firstembodiment of the present invention.

FIG. 3 is a flowchart showing an operation of determining a masterbranch in the first embodiment of the present invention.

FIG. 4 is a block diagram showing a structure of a conventionaldiversity receiving apparatus.

FIG. 5 is a view showing a structure of a conventional OFDM symbolperiod.

FIG. 6 is a view showing extraction of an OFDM signal in an effectivesymbol period by conventional quadrature detection units.

REFERENCE MARKS IN THE DRAWINGS

-   101 a, 101 b antenna-   102 a, 102 b tuner-   103 a, 103 b A/D converter-   104 a, 104 b quadrature detection unit-   105 a, 105 b FFT calculator-   106 a, 106 b demodulation unit-   107 master branch determination unit-   108 timing adjustment unit-   109 synthesis unit-   110 transmission parameter storage unit-   111 deinterleave unit-   112 demapping unit-   113 bit deinterleave unit-   114 error correction unit

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

A diversity receiving method of an embodiment of the present inventionwill be described as follows, with reference to drawings.

First Exemplary Embodiment

First of all, an example of the structure of the receiving apparatus forrealizing the diversity receiving method of the present invention willbe described as follows.

FIG. 1 is a block diagram showing the structure of the receivingapparatus of an embodiment of the present invention.

In FIG. 1, antenna 101 a receives and converts a braodcasting wavetransmitted from a broadcast station into an electric signal and outputsit. Tuner 102 a extracts a signal at specific frequencies from thesignal received from antenna 101 a, and converts the extracted signalinto a signal in the base band or at specific frequencies. Tuner 102 aalso outputs information about the average electric power of thereceived signal and information about fluctuation in the electric power.

A/D converter 103 a converts an analog signal received from the tunerinto a digital signal.

Quadrature detection unit 104 a detects the OFDM signal, and calculatesand corrects the frequency error between the transmitting signal and thefrequency reference signal owned by demodulation unit 106 a, therebydetermining the transmission mode of the OFDM signal and the length ofthe guard interval. Quadrature detection unit 104 a then extracts thesignal in the effective symbol period from the OFDM signal, and outputsthe extracted signal. Quadrature detection unit 104 a also outputs asignal indicating synchronization establishment.

Each OFDM signal is made up of a signal in the guard interval and asignal in the effective symbol period. The signal in the guard intervalis a signal at the tail of the effective symbol period and is added asit is in the guard interval which is ahead of the effective symbolperiod. In other words, the signal in the guard interval is a copy ofpart of an OFDM signal, so that the OFDM symbol period (the effectivesymbol period+the guard interval) contains the same signal repeatedlyappearing at different positions. Calculating the correlation value ofthe OFDM signal in the time direction indicates that a high correlationis obtained when the information indicating the transmission mode andthe information indicating the guard interval are detected, therebydetermining the transmission mode and the guard length.

Quadrature detection unit 104 a extracts only the signal correspondingto the effective symbol period, and transmits it to FFT calculator 105 awhich is provided at a later stage. Quadrature detection unit 104 aregards the determination of the transmission mode and the guard lengthas the establishment of synchronization, and transmits a signalindicating the synchronization establishment to the subsequent stage.

FFT calculator 105 a subjects the signal in the time domain of the OFDMsymbol period received from quadrature detection unit 104 a to an FFTcalculation process so as to convert the signal into a signal in thefrequency domain. FFT calculator 105 a also outputs a timing signal sothat it is possible to determine to which OFDM carrier each FFTcalculation result corresponds at a process at a later stage.

Demodulation unit 106 a demodulates a TMCC (Transmission andMultiplexing Configuration and Control) signal inserted in the OFDMsignal so as to obtain various parameter information of the OFDM signal.The transmission parameter information includes transmission mode,carrier modulation system, coding rate, hierarchical information, andinterleave length.

Demodulation unit 106 a also extracts pilot signals located in the OFDMsignal at regular intervals in the frequency and time directions.Demodulation unit 106 a then compares the extracted pilot signals withthe reference value (known amplitude and phase) so as to calculate thetransmission characteristics (the degree of differences in amplitude andphase) of the carrier containing the pilot signals based on changes inamplitude and phase. Demodulation unit 106 a then interpolates thetransmission characteristics of the carrier containing the pilotsignals, which have been calculated by the pilot extraction unit, in thetime and frequency directions. Thus, demodulation unit 106 a calculatesand outputs the estimate of the transmission characteristics of all theOFDM carriers. The interpolation is performed by using the transmissioncharacteristics of the pilot carriers and in such a manner that thetransmission characteristics increase or decrease in accordance with thenumber of data carriers existing between the pilot carriers, or are keptat an average value. Demodulation unit 106 a then divides the signalreceived from FFT calculator 105 a by the estimate of the transmissioncharacteristics so as to found a complex signal based on the divisionresult and outputs it to timing adjustment unit 108.

Demodulation unit 106 a also outputs, to timing adjustment unit 108,electric power information indicating the amount of electric power ofeach OFDM carrier and amplitude information indicating the amount ofamplitude of each OFDM carrier which are calculated by using thetransmission characteristics estimated from the pilot signals.Demodulation unit 106 a also calculates and outputs the average amountof noise contained in the received signal. The average amount of noisecan be calculated from the average of square values of the distancesbetween received signal points (the complex signals after the division)and the mapping points.

Components 101 b to 106 b have the same structures as components 101 ato 106 a, respectively, so that their individual description will beomitted.

Master branch determination unit 107 determines the optimum masterbranch by using the signals indicating the synchronization establishmentreceived from quadrature detection units 104 a and 104 b, and theinformation about average electric power and the information about thefluctuation in the electric power received from tuners 102 a and 102 b.Master branch determination unit 107 then transmits a signal indicatingthe branch determined to be the master branch to timing adjustment unit108.

The operation to determine the master branch by master branchdetermination unit 107 will be described later.

Timing adjustment unit 108 temporarily buffers a complex signal receivedfrom demodulation unit 106 a (or 106 b) of the master branch determinedby master branch determination unit 107. Timing adjustment unit 108 thenprovides a delay in consideration of the timing difference between thereceived signals. The delay can be as long as, for example, the guardinterval or the OFDM symbol period. Timing adjustment unit 108 alsobuffers the complex signal received from the slave branch, and takes itout at the same timing as the signal of the master branch.

Timing adjustment unit 108 detects the timing difference between thesignal of the master branch and the signal of the slave branch from thedifference between the timing signal of the master branch and the timingsignal of the slave branch received from FFT calculator 105 a (or 105b).

When the delay time is as long as the OFDM symbol period, it isnecessary for timing adjustment unit 108 to obtain a signal indicating asymbol number in addition to the timing signal so as to prevent thesynthesis of OFDM signals at different positions in the sequence. It isof course possible for timing adjustment unit 108 to receive and comparethe symbol numbers even when the delay time is as long as the guardinterval. The reason to make the delay time about as long as the guardinterval is that the maximum timing difference in extracting an OFDMsignal by quadrature detection unit 104 a (or 104 b) is assumed to beabout as long as the guard interval. Synthesizing signals inconsideration of symbol number can prevent the synthesis of signalscontained in different symbols, even when a timing difference largerthan expected is caused for some reasons.

Timing adjustment unit 108 outputs the complex signals that havesubjected to the timing adjustment to synthesis unit 109. Timingadjustment unit 108 may have a memory for temporarily storing thecomplex signals received from the master and slave branches so as tomake the memory perform the timing adjustment.

Synthesis unit 109 receives from timing adjustment unit 108 the complexsignals subjected to the timing adjustment, the electric powerinformation and the amplitude information. Synthesis unit 109 thensynthesizes the complex signals obtained from demodulation units 106 aand 106 b. In the synthesis, synthesis unit 109 performs a weightingprocess to the complex signals and sums them up based on the electricpower information and amplitude information of the carriers containingthe received signals, which have been calculated by demodulation units106 a and 106 b. The electric power information and the amplitudeinformation may be quantized values.

Synthesis unit 109 outputs the complex information and reliabilityinformation of synthesized received signal points to deinterleave unit111 provided at a later stage. As the reliability information, thelargest value of the power information about the signals used for thesynthesis is outputted.

Transmission parameter storage unit 110 receives a signal indicatingsynchronization establishment from each of quadrature detection units104 a and 104 b, and also receives a signal indicating transmissionparameters from each of demodulation units 106 a and 106 b. Transmissionparameter storage unit 110 then outputs the transmission parameterinformation received from a branch which has establishedsynchronization, whether it is the master branch or the slave branch, tolater-stage blocks (deinterleave unit 111, demapping unit 112, bitdeinterleave unit 113, and error detection unit 114). Thus transmittingthe transmission parameters to the later-stage blocks whether it is inthe master branch or the slave branch by transmission parameter storageunit 110 allows the synthesized complex signal to be decoded even whenthe master branch has been replaced.

Since transmission parameters other than symbol number do not changeoften in this case, it is alternatively possible to maintain theparameters obtained after channel selection. It is also possible tocheck changes in the parameters periodically, and to update any changes.

Transmission parameter storage unit 110 can receive a signal indicatingthat a TMCC signal has been decoded from demodulation unit 106 a (or 106b), instead of receiving the signal indicating synchronizationestablishment from each of quadrature detection units 104 a and 104 b,and can output the transmission parameter information received from abranch having a decoded TMCC signal to the later-stage blocks.

As an approach to transmit the transmission parameter information to thelater-stage blocks (deinterleave unit 111, demapping unit 112, bitdeinterleave unit 113 and error correction unit 114), it is possible tomultiply the transmission parameter information in a data signal line,instead of providing a separate signal line. This approach is possiblebecause although the number of output signals of FFT calculators 105 aand 105 b corresponds to the number of points in the FFT calculation,the total number of the OFDM carriers is smaller than the number ofpoints in the FFT calculation, thereby generating an ineffective periodin the data. For example, when the transmission parameters are mode 3,the number of FFT points is 8192, and the total number of the OFDMcarriers is 5717. This means that 2575 (8192-5717) data points are notneeded at the later stages. In a structure where 8192 data points areoutputted sequentially from the FFT calculators, part of the ineffectiveperiod in the data can be replaced with the transmission parameters andextracted at a later-stage block.

Deinterleave unit 111 sorts the complex signals in the synthesizedreceived signal points received from synthesis unit 109 in the frequencyand time directions. The sorting method is defined in advance and thesort is performed to clear the sort done on the transmitting side so asto restore the original order. Deinterleave unit 111 receivesinformation about the interleave length and segment number for eachhierarchical level and information about the presence or absence of ahierarchical level of partial reception from transmission parameterstorage unit 110. Deinterleave unit 111 then sorts data in accordancewith the parameter of the length of the time interleave of eachhierarchical level.

Demapping unit 112 restores the bit data of the signals based on theinformation of the received signal points obtained from deinterleaveunit 111. The demodulation is performed by determining the number ofsegments in each hierarchical level and the carrier modulation systemfrom the transmission parameters outputted from transmission parameterstorage unit 110. The restoration of the bit data is performed on theassumption that the code string assigned to the mapping point closest toa received signal point is the transmitting code string. For example,when a received signal is carrier modulated at 16 QAM, the bit data isrestored in accordance with the rule shown in FIG. 2. Demapping unit 112calculates for each bit the shortest distance between a mapping pointwhere the transmission code is 0 and a mapping point where thetransmission code is 1 with respect to a received signal point, andtransmits a value determined by the distance as a likelihood (0likelihood or 1 likelihood) to the error correction unit at the laterstage (called “soft decision”).

Demapping unit 112 then corrects the calculated likelihood by using thereliability information received from synthesis unit 109 throughdeinterleave unit 111. When synthesis unit 109 determines that thereliability is low, demapping unit 112 corrects the likelihood to 0 (0likelihood and 1 likelihood are equal to each other). On the other hand,when synthesis unit 109 determines that the reliability is high,demapping unit 112 outputs the calculated likelihood with nocorrections. Alternatively, demapping unit 112 outputs the reliabilityvalue, which is the electric power information and amplitude informationof the carrier received from synthesis unit 109, after multiplying thereliability value with the likelihood calculated earlier.

As described above, demapping unit 112 outputs the bit data streamcalculated from the information of the received point obtained fromsynthesis unit 109, and the calculated likelihood for each bit.

Bit deinterleave unit 113 then sorts the outputs of demapping unit 112.The sorting method is defined in advance, and the sort is performed toclear the sort done on the transmitting side so as to restore theoriginal order.

Error correction unit 114 performs error correction, based on theinformation about segment number and carrier modulation system and theinformation about error correction coding rate, which are received fromtransmission parameter storage unit 110, and by using the bit datastream received from bit deinterleave unit 113 and the information aboutthe likelihood of each bit data. In such error correction, an errorcorrecting method called “viterbi decoding” is often used, and is oftencombined with “Reed-Solomon error correction code”; however, this is notthe only error correction possible and any other error corrections canbe used as long as the aforementioned likelihood is used.

The structure of the diversity receiving apparatus of the firstembodiment of the present invention has been thus described hereinabove.

The operation for the diversity receiving apparatus to determine themaster branch will be described as follows with reference to theflowchart of FIG. 3.

First of all, when the diversity receiving apparatus of the presentinvention is powered on or selects a channel (Step S001), master branchdetermination unit 107 determines the master branch by using the signalindicating synchronization establishment received from each ofquadrature detection units 104 a and 104 b (Step S002). Morespecifically, master branch determination unit 107 compares the timeswhen the signals indicating synchronization establishment are receivedfrom quadrature detection units 104 a and 104 b, and selects as themaster branch the branch from which the signal indicatingsynchronization establishment has been received earlier than the otherafter the power on or the channel selection.

As an alternative approach to determine the master branch, master branchdetermination unit 107 can use the information about the averageelectric power of a received signal and the information aboutfluctuation in the electric power obtained from tuners 102 a and 102 b.More specifically, the branch having the highest average electric powerof the received signal can be determined to be the master branch, or thebranch performing stable reception with the least fluctuation in theelectric power of the received signal can be determined to be the masterbranch, thereby selecting a branch which is considered to be in the bestreceiving condition as the master branch. By using the signal of thebranch determined to be the master branch as the reference, timingadjustment unit 108 at the subsequent stage can perform timingadjustment.

As another alternative approach to determine the master branch, masterbranch determination unit 107 can receive data indicating the averageamount of noise from demodulation units 106 a and 106 b, anddiscriminate the branch having the least average amount of noise.

In the case of a diversity receiving apparatus having a plurality ofantennas with directional characteristics, the branch having an antennawith directional characteristics in the direction of a transmittingstation can be determined to be the master branch. In this case,however, determining which antenna is in the direction of thetransmitting station requires the following procedure: to combine a GPS(Global Positioning System) capable of calculating the positionalrelation between the diversity receiver and the transmitting stationwith a gyro sensor or the like capable of detecting the direction of thediversity receiver; and to input the positional and directionalinformation from these devices to master branch determination unit 107of the diversity receiving apparatus of the present invention. Forexample, in the case of a diversity receiver used in a car navigationsystem, the position and direction of a transmitting station can befound by the GPS and sensor in the car navigation system. Therefore, theinformation can be inputted from the car navigation system to masterbranch determination unit 107 of the diversity receiving apparatus ofthe present invention to discriminate the antenna having directionalcharacteristics in the direction of the transmitting station, therebydetermining the master branch. In the case of a car navigation system, amobile phone or a mobile computer with a GPS, the direction of thediversity receiving apparatus is frequently changed according to thedirection of the car or the user. This makes it possible to determinethe master branch by storing the past information about the direction ofthe diversity receiving apparatus in a memory and by using the meanvalue to calculate the direction in which the car or the user has stayedlongest for a certain period.

When the diversity receiving apparatus of the present invention iscombined with a car navigation system, a mobile phone and a mobilecomputer, the master branch can be determined by another method. Forexample, a memory or HDD in these devices can store the master branchthat was receiving a signal; the branch that was having the highestaverage electric power of the received signal; or the branch that washaving the least fluctuation in the electric power of the receivedsignal the last time the car or the user passed through a certain point.Then, the information is outputted to master branch determination unit107 of the diversity receiving apparatus of the present invention whenthe car or the user passes through the same point, so that master branchdetermination unit 107 can determine the branch to be the master branch.It is alternatively possible to use a recording medium such as a CD-ROM,DVD or SD memory card to store the information in order to determine themaster branch.

As further another alternative approach to determine the master branch,the diversity receiving apparatus of the present invention can becombined with a car navigation system, a mobile phone or a mobilecomputer having a communication unit capable of bidirectionalcommunication. The relationship between the point at which the diversityreceiving apparatus exists and the branch to be the master branch areaccumulated in a server communicable with the car navigation system orthe like, and the accumulated information is used in determining themaster branch. In this case, master branch determination unit 107 of thediversity receiving apparatus of the present invention can upload thelatest information via the car navigation system or the like eithercontinuously or at regular intervals. This allows the car navigationsystem or the like to determine the master branch based on the latestinformation when the car navigation system or the like passes throughthe same point later.

Master branch determination unit 107 transmits a signal indicating themaster branch thus determined to timing adjustment unit 108. Timingadjustment unit 108 temporarily buffers a complex signal received fromdemodulation unit 106 a (or 106 b) of the master branch according to thesignal indicating the master branch, takes the complex signal out at thesame timing as the complex signal received from demodulation unit 106 b(or 106 a) of the slave branch, and outputs these complex signals tosynthesis unit 109.

Although the master branch is determined in this manner, in the case ofa car navigation system, a mobile phone, a mobile computer and the like,the directions of the antennas of the diversity receiving apparatuschange frequently in accordance with the direction of the transmittingantenna, so that the branches are not in the same receiving condition.When the master branch gets into bad receiving condition, it becomesimpossible to receive the timing signal from the master branch, which isused as the reference timing in adjustment unit 108. This may cause atiming difference in the synthesis. Furthermore, the antenna wire of themaster branch may be broken due to corrosion or an accident. In view ofsuch cases, master branch determination unit 107 determines a nextcandidate for the master branch after the determination of the masterbranch (Step S003).

The determination of the next candidate for the master branch isperformed in the same manner as the determination of the master branchdescribed above, and a signal indicating the branch that has beensearched for as the next candidate for the master branch is stored in abuffer unit or a memory unit provided either inside or outside of masterbranch determination unit 107.

When the initially selected master branch causes a loss ofsynchronization due to the aforementioned causes during the reception ofa broadcasting wave (Step S004), master branch determination unit 107transmits the signal indicating the branch determined to be the nextcandidate for the master branch to timing adjustment unit 108. Timingadjustment unit 108 temporarily buffers a complex signal received fromdemodulation unit 106 b (or 106 a) of the next candidate for the masterbranch, takes out the complex signal at the same timing as the complexsignal received from demodulation unit 106 a (or 106 b) of the slavebranch, and outputs these complex signals to synthesis unit 109.

When the car or the user enters a tunnel while using the car navigationsystem or the mobile phone, the broadcasting wave cannot be received fora certain period. In such a case, even if the next candidate for themaster branch is already determined as described above, the nextcandidate for the master branch may not be able to receive thebroadcasting wave, either. In this case, master branch determinationunit 107 searches for active branches (Step S005) and selects a masterbranch from among the active branches (Step S006). When finding noactive branch, master branch determination unit 107 determines the nextcandidate for the master branch again, and outputs the signal indicatingthe next candidate for the master branch to timing adjustment unit 108.Timing adjustment unit 108 waits for the master branch to restore thereception of the broadcasting wave (Step S007).

When no active branch is found at Step S005, master branch determinationunit 107 determines the branch that was the last to be insynchronization (immediately before the occurrence of receptioninterruption) to be the master branch, based on the past receivingcondition. This makes it possible to prepare as the master branch thebranch which is expected to be in the best receiving condition when thereception is resumed. As an alternative approach to determine the masterbranch, it is possible to compare the average electric power received orfluctuation in the electric power received within a certain period inthe past among the branches, based on the past receiving condition, andthen to select the branch having the highest average electric powerreceived or the branch having the least fluctuation in the electricpower received. For the determination of the master branch based on thepast conditions such as synchronization establishment, master branchdetermination unit 107 is required to have a buffer unit or a memoryunit for storing the past conditions such as synchronizationestablishment either inside or outside it.

Since the condition of receiving a broadcasting wave changes by theminute while the car or the user is moving, master branch determinationunit 107 may determine the next candidate for the master branchperiodically and output the signal indicating the master branchperiodically to timing adjustment unit 108, and timing adjustment unit108 may change the master branch periodically in response to this,thereby adjusting the timing between the branches. This achievesflexible replacement of the master branch in accordance with thecondition of receiving a broadcasting wave. In this case, the timing toreplace the master branch can be determined freely depending on the typeof the diversity receiving apparatus, such as a car navigation system, amobile phone or a mobile computer.

As described hereinbefore, the diversity receiving apparatus of thepresent invention can replace the master branch with the optimum branchwhen the master branch gets into bad receiving condition or when theantenna signal wire on the master side comes off or breaks, therebyachieving synchronization of the timing between the master and theslave.

Transmission parameters including information such as transmission mode,carrier modulation system, coding rate, hierarchical information andinterleave length are received by transmission parameter storage unit110 from demodulation unit 106 a (or 106 b) of the branch which hasestablished synchronization, whether it is the master branch or theslave branch, and are outputted to deinterleave unit 111, demapping unit112, bit deinterleave unit 113, and error correction unit 114. Thisallows the parameters used for demodulation to be transmitted todeinterleave unit 111 and other units whether it is in the master branchor the slave branch, when the master branch is going to be replaced dueto the worsening receiving condition.

The first embodiment has thus described a diversity receiving apparatushaving two branches with reference to FIG. 1; however, the apparatus mayhave three, four or any other number of antennas.

In addition, the present invention is not limited to the transmissionsystem, and can be a multi-carrier transmission system with OFDM or asingle carrier transmission system with VSB (VESTIGIAL SIDE BAND). Thereis also no limitation to the digital modulation system and can be 8 QAM,16 QAM, 32 QAM, 64 QAM, 256 QAM, QPSK (Quadrature Phase Shift Keying) orthe like.

The diversity receiving apparatus and method of the present inventionhave thus been described in the first embodiment. Receiving apparatusessuch as a personal computer (PC), a television, a video recorder and aSTB (Set-top Box) containing or connected with antennas can have therespective receiving methods incorporated therein as software, and makethe CPU (Central Processing Unit) of the PC or the like process andexecute the software, thereby achieving diversity reception for theselection and synthesis.

In order to make the CPU in the PC or the like process and execute thereceiving method of the first embodiment, these methods can be stored toa recording medium such as a CD-ROM, as a program or data equivalent toa program in such a manner as to be processable by the CPU. As a result,the aforementioned methods can be achieved on a PC having a reader.

INDUSTRIAL APPLICABILITY

The present invention is useful for maintaining the performance ofreception in a diversity receiving apparatus.

1. A diversity receiving apparatus for receiving signalscarrier-modulated by a digital multilevel modulation system by aplurality of branches and for demodulating the signals, the diversityreceiving apparatus comprising: a plurality of demodulation units eachprovided for each of the plurality of branches, the plurality ofdemodulation units each outputting complex information indicating asignal point of each of the signals received by the plurality ofbranches; a master branch determination unit for determining a masterbranch which is used as a reference in synchronizing an output timingamong symbols of the signals received by the plurality of branches, andfor outputting a signal indicating a branch determined to be the masterbranch; a timing adjustment unit for receiving the signal indicating thebranch determined to be the master branch from the master branchdetermination unit, and for adjusting a timing of synthesizing thesignals received by the plurality of branches by synchronizing theoutput timing between a symbol of the complex information received fromthe demodulation unit of the master branch and a symbol of the complexinformation received from the demodulation unit of a branch other thanthe master branch out of the plurality of antennas; and a synthesis unitfor synthesizing the signals received by the plurality of branches byusing the complex information that has been timing-adjusted by thetiming adjustment unit.
 2. The diversity receiving apparatus of claim 1further comprising: a transmission parameter storage unit, wherein theplurality of demodulation units each extract a transmission parametercontaining information necessary for demodulation from each of thesignals received by the plurality of branches, and each output thetransmission parameter in addition to the complex signal, and thetransmission parameter storage unit stores the transmission parametersoutputted from the plurality of demodulation units.
 3. The diversityreceiving apparatus of claim 1 further comprising: a plurality ofquadrature detection units each provided for each of the plurality ofbranches, the plurality of quadrature detection units each outputting asignal indicating synchronization establishment of each of the signalsreceived by the plurality of branches, wherein the master branchdetermination unit determines a branch having the quadrature detectionunit that is the first to input the signal indicating synchronizationestablishment to the master branch determination unit to be the masterbranch.
 4. The diversity receiving apparatus of claim 1 furthercomprising: a plurality of tuners each provided for each of theplurality of branches, the plurality of tuners each extracting a signalat specific frequencies from each of the signals received by theplurality of branches, and each outputting information about averageelectric power of each of the signals received by the plurality ofbranches, wherein the master branch determination unit determines abranch having a highest average electric power of the signal received tobe the master branch.
 5. The diversity receiving apparatus of claim 1further comprising: a plurality of tuners each provided for each of theplurality of branches, the plurality of tuners each extracting a signalat specific frequencies from each of the signals received by theplurality of branches, and each outputting information about fluctuationin electric power of each of the signals received by the plurality ofbranches wherein, the master branch determination unit determines abranch having least fluctuation in the electric power of the signalreceived to be the master branch.
 6. The diversity receiving apparatusof claim 1, wherein the plurality of demodulation units each calculatean average amount of noise from each of the signals received by theplurality of branches and each output the average amount of noise; andthe master branch determination unit determines a branch having leastaverage amount of noise outputted from the demodulation unit to be themaster branch.
 7. The diversity receiving apparatus of claim 1, whereinthe plurality of branches each have an antenna with directionalcharacteristics: and the master branch determination unit determines abranch having an antenna with directional characteristics in a directionof a transmitting station to be the master branch.
 8. The diversityreceiving apparatus of claim 7 further comprising: a GPS capable oflocating a position of a receiving apparatus; and a gyro sensor capableof locating a direction of a receiving apparatus, wherein the masterbranch determination unit selects an antenna with directionalcharacteristics in a direction of a transmitting station based on theGPS and the gyro sensor, and determines the antenna to be the masterbranch.
 9. The diversity receiving apparatus of claim 1 furthercomprising: a GPS capable of locating a position of the diversityreceiving apparatus; and a communication unit capable of bidirectionalcommunication, wherein the master branch determination unit transmitspositional information of the diversity receiving apparatus located bythe GPS to a server having information about master branchdetermination, and receives the information about master branchdetermination from the server, thereby determining a branch having anantenna with directional characteristics in a direction of atransmitting station to be the master branch.
 10. The diversityreceiving apparatus of claim 1 further comprising: a reader capable ofreading a recording medium which stores the information about masterbranch determination, wherein the master branch determination unitdetermines a branch having an antenna with directional characteristicsin a direction of a transmitting station to be the master branch, basedon the information about master branch determination that has been readfrom the recording medium by the reader.
 11. The diversity receivingapparatus of claim 1, wherein the master branch determination unitdetermines a new master branch by selecting the new master branch amongbranches that can receive signals when the master branch gets into badreceiving condition and it becomes impossible to detect symbolsynchronization.
 12. The diversity receiving apparatus of claim 11,wherein the master branch determination unit stores receiving statusdata before the master branch gets into bad receiving condition; anddetermines the new master branch based on the receiving status data. 13.The diversity receiving apparatus of claim 12, wherein the master branchdetermination unit stores, as the receiving status data before themaster branch gets into bad receiving condition, the signals eachindicating synchronization establishment of each of the plurality ofbranches; and determines a branch that is the last to input the signalindicating synchronization establishment to the master branchdetermination unit before the master branch gets into bad receivingcondition to be the new master branch.
 14. The diversity receivingapparatus of claim 12, wherein the master branch determination unitstores, as the receiving status data before the master branch gets intobad receiving condition, average electric power received by each of theplurality of branches; and determines a branch that has had highestaverage electric power received before the master branch gets into badreceiving condition to be the new master branch.
 15. The diversityreceiving apparatus of claim 12, wherein the master branch determinationunit stores, as the receiving status data before the master branch getsinto bad receiving condition, fluctuation in electric power received byeach of the plurality of branches; and determines a branch that has hadleast fluctuation in the electric power received before the masterbranch gets into bad receiving condition to be the new master branch.16. The diversity receiving apparatus of claim 1, wherein the masterbranch determination unit determines a new master branch even when itbecomes impossible to detect symbol synchronization because the masterbranch gets into bad receiving condition, and there is no other branchthat can receive a signal.
 17. The diversity receiving apparatus ofclaim 2, wherein the transmission parameter storage unit stores thetransmission parameter that is the first to be outputted from theplurality of demodulation units after channel selection, thetransmission parameter being used to demodulate the signal received by acorresponding one of the plurality of branches.
 18. The diversityreceiving apparatus of claim 2, wherein the transmission parameterstorage unit stores the transmission parameter that is the first to beoutputted from the plurality of demodulation units after the receptionis resumed, the transmission parameter being used to demodulate thesignal received by a corresponding one of the plurality of branches. 19.A diversity receiving method for receiving signals carrier-modulated bya digital multilevel modulation system by a plurality of branches andfor demodulating the signals, the diversity receiving method comprising:a demodulation step for outputting complex information indicating signalpoints of the signals received by the plurality of branches; and amaster branch determination step for determining a master branch whichis used as a reference in synchronizing an output timing among symbolsof the signals received by the plurality of branches, and for outputtinga signal indicating a branch determined to be the master branch.
 20. Thediversity receiving method of claim 19, wherein the master branchdetermination step comprises: a step for determining a next candidatefor the master branch; and a step for replacing a current master branchwith the next candidate for the master branch when the current masterbranch gets into bad receiving condition.